Vol .23,No .72002年7月
C~INESE JOURNAL OF SEMICONDUCTORS
July =
================================================================,2002
%Project supported by National Natural Science Foundation of China (No .69825107D ,NSFC -RGC Joint Program (Nos .NSFC 5001161953and
N -~KU 028/00D
Shen Xiaoming male ,Was born in 1965,PhD candidate .NoW he Works on the researches of GaN materials and related devices .Yang ~ui
male ,Was born in 1961,professor .~is research interests are in semiconductor materials and optoelectronic devices .
Received 29November 2001,revised manuscript received 23January 2002
c 2002The Chinese Institute of Electronics Etching behavior of GaN /GaAs (001D Epilayers
Grown by MOVPE
%
Shen Xiaoming 1,2,Feng Zhihong 1,Feng Gan 1,Fu Yi 1,Zhang Baoshun 1
,
Sun Yuanping 1,Zhang Zehong 1and Yang ~ui
1
(1State Key La}oratory on 1ntegrated O ptoelectronics ,1nstitute of Semiconductors ,
The Chinese academy of Sciences ,Beijing
100083,China D
(2Department of Physics ,GuangLi Uni ersity ,Nanning
530004,China D
Abstract :W et etching characteristics of cubic GaN (c -GaN D thin films groWn on GaAs (001D by metalorganic vapor phase epita X y (MOVPE D are investigated .The samples are etched in ~Cl ,~3PO 4,KO~a f ueous solutions ,and molten KO~at temperatures in the range of 90~300 .It is found that different solution produces different etch figure on the surfaces of a sample .KO~-based solutions produce rectangular pits rather than s f uare pits .The etch pits elongate in [110]direction ,indicating asymmetric etching behavior in the tWo orthogonal <110>directions .An e X planation based on relative reactivity of the various crystallographic planes is employed to interpret f ualitatively the asymmetric etching behavior .In addition ,it is found that KO~a f ueous solution Would be more suitable than molten KO~and the tWo acids for the evaluation of stacking faults in c -GaN epilayers .K ey wor d s :cubic GaN ;MOVPE ;Wet etching ;asymmetry EEA CC :2520D ;2550E CLC n um ber :TN 405
D oc um ent co d e :A
Article ID :0253-4177(2002D 07-0707-06
1I ntro du ction
Considerable progress has been made recently
in the areas of groWth ,dry etching ,and doping of the I -nitrides and their ternary alloys .This has resulted in GaN -based blue light emitting diodes and laser diodes
[1,2]
.Despite the great success
achieved so far ,there are still many problems to be solved .The epita X ial layers invariably contain high densities of defects resulting from the large lattice mismatch betWeen epilayers and substrates
[3]
.
Therefore ,the availability of reliable and f uick methods to investigate the defects in GaN is of
great interest .
W et -chemical etching is a useful method for surface defect investigation and has been Widely used in I -V compound semiconductors .~oWever ,there has been little success in the development of Wet etching techni f ues for I -N nitrides Which
have e X cellent chemical stability [4]
.Mileham et al .[5]reported the etching of AlN defective single
碳氟化钾crystals in KO~-based solutions at etch tempera-tures in the range of 23~80 .They found de-creasing etch rates With increasing crystal f uality ,as the reactions occurred favorably at grain bound-aries and defect sites .InN in a f ueous KO~solu-tions Was reported to etch at a feW nm /min at
60C[6].For GaN,there Were several early reports of Wet etching in NaO~that hindered by formation of an insoluble gallium hydroXide(GaO~)coat-ing[7].Others have reported that~3PO4can remove GaN at a very sloW rate.Although GaN can not be etched With various conventional chemical solu-tions at room temperature,some hot etching solu-tions,such as hot~3PO4(Z15C)and molten KO~ (360C)have been able to etch pits at defect sites on the c-plane of GaN epilayers groWn on sap-phire[8,9].More recently,photoenhanced Wet etch-ing of GaN has been regarded as a means of grea
tly improving the chemical reactivity of GaN at room temperature.Ultraviolet(UV)illumination is used to generate electron-hole pairs at the semiconduc-tor surface,Which enhance the oXidation and reduc-tion reactions Within an electrochemical cell.The photoelectrochemical(PEC)etching can produce anisotropic[10],dopant selective[11],and uniform etching[1Z,13].Most of the PEC etching eXperiments Were performed at room temperature using KO~ agueous solution as electrolyte With no bias.Under some special conditions,the dislocation microstruc-ture in the GaN can be revealed by selectively re-moving material betWeen the dislocations,resulting in Whisker formation[14].Up to noW,hoWever,there are no reports for Wet etching of c-GaN epilayers groWn on GaAs(001)substrates.
In this study,in order to investigate prelimi-narily the etching behavior and defect revealing,We performed Wet chemical etching on(001)surfaces of MOVPE groWn c-GaN epilayers. e etched c-GaN epilayers by using~Cl,~3PO4,KO~agueous solutions and molten KO~in the temperature range of90~300C.Asymmetric etching behavior in the tWo orthogonal<110>directions Was ob-served.
2Experimental procedure
GaN epilayers used for Wet etching Were groWn on GaAs(001)substrates in a horizontal MOVPE re
actor operating at loW pressure (104Pa).TEGa and N~3Were used as the precur-sors for Ga and N,respectively.The GaN epilayers Were groWn in~Z ambient at8Z0C for1h after groWing the GaN buffer layers at550C.All the films Were0.6pm in thickness.The unintentionally doped GaN films Were measured to be n-type With a carrier concentration of~1017cm3.Typical full Width at half of maXimum(F ~M)of double-crystal X-ray diffraction(00Z)c scan,from GaN films Was about Z5/.Our plan-vieW TEM studies of the as-groWn epilayers indicated a defect density in the range of109~1010cm Z.Commercial36%~Cl, 85%~3PO4,and5M KO~agueous solution Were heated to etching temperatures90,140and1Z0C, respectively.The samples Were immersed in the chemical etchants,Which Were not stirred during etching.Molten KO~etching Was also performed on the c-GaN films.KO~Was melted in a platinum beaker and the etch temperature Was about300C. JEOL-6301F scanning electron microscopy(SEM) Was employed to eXamine the surface morphology of the as-groWn and etched GaN samples.The plan-vieW TEM specimen Was prepared by mechan-ical thinning folloWed by ion milling.The GaN epi-layer Was thinned from the substrate side so that only the defects near the surface region Were im-aged.Plan-vieW TEM observation Was carried out on a~itachi~-800TEM With an accelerating volt-age of Z00kV.
3Results and discussion
Figure1shoWs plan-vieW SEM images of as-groWn c-GaN epilayer and layers Wet etched in sev-eral acid and base solutions at temperatures in the range of90~300C for5min.The surfaces of the samples are mirror-like before etching.~oWever,as can be seen in Fig.1(a),ridges and pits on the sur-faces can be observed by SEM.Figure1(b)shoWs a SEM image of a sample after it Was etched in commercial36%~Cl at90C.SaWtoothed etch fig-ures developed on the sample surface.Figure1(c)
807半导体学报Z3卷
iS for a Sample immerSed in commercial85%phoS-phoric acid(~3PO4D at140 for5min.After it WaS etched in thiS etchant,Spindle-Shaped etch pitS developed on the Sample*S Surface.TheSe pitS elon-gate in[110]direction.A typical SEM micrograph of the GaN Surface after molten KO~etched iS ShoWn in Fig.1(d D.AS it can be Seen,molten KO~ produced rectangular etch pitS on the Sample Sur-face.It iS Well knoWn that molten KO~haS been uSed SucceSSfully in evaluation of diSlocationS in Wurtzite GaN groWn on Sapphire.In our etching experiment,hoWever,molten KO~produced reSid-ual complex on the Surface of the Sample.In the vieW point of revealing defectS,molten KO~iS not Suitable for evaluating the defectS in c-GaN epilay-erS
.
Fig.1SEM imageS of SurfaceS of aS-groWn GaN and GaN epilayerS after Wet chemical etching in acid and baSe SolutionS at temperatureS up to300 for5min(a D AS-groWn;(b D36%~Cl,90 ;
(c D85%~3PO4,140 ;(d D Molten KO~,300
Figure2ShoWS the SEM imageS of the Sur-faceS of GaN etched in5M KO~agueouS SolutionS at120 With different etch timeS.AS in the caSe of molten KO~etching,agueouS KO~SolutionS can alSo produce rectangular pitS on the SurfaceS of c-GaN epilayerS.After being etched for5min,elon-gated etch pitS Were viSible on the SurfaceS of the GaN epilayerS,aS ShoWn in Fig.2(a D.The etch fig-ureS greW in Size and merged into each other With the increaSing of etch time,aS can clearly be Seen in Fig.2(b D and(c D.In addition,StepS both in[110] and[110]directionS can be obServed on the Sur-faceS,eSpecially in Fig.2(c D.
AS deScribed above,different chemical Solu-tionS produce different etch figureS on the GaN SurfaceS.~ot~Cl developS SaWtoothed figureS Which align the edgeS of original pitS(Fig.1(a D and(b D D.~ot~3PO4produceS Spindle-Shaped etch pitS on the Surface.On the other hand,KO~-baSed SolutionS produce rectangular etch pitS,Which are conSiStent With the cryStallographic Symmetry of c-GaN With zinc-blende Structure.ThiS iS different from the Wet chemical etching of Wurtzite GaN groWn on Sapphire,in Which both acid and baSe So-lutionS produce hexagonal etch pitS,Which reflect the cryStallographic Symmetry of Wurtzite GaN.
硅钢片密度
907
7期Shen xiaoming et al.,Etching behavior of GaN/GaAS(001D EpilayerS GroWn by
Fig.2SEM images Of GaN epilayer etched by K0~
agueOus sOlutiOn With different etch times(a)
5min;(b)10min;(c)20min
~ere We Only try tO discuss the rectangular etch pits prOduced by mOlten K0~and agueOus K0~ sOlutiOn.
It is Well knOWn that the Zinc-blende structure exhibits nO pOlarity in the<001>directiOns differ-ences in etching behaviOr are nOt expected amOng the variOus{001}surfaces.In certain etchants hOWever{111}facets develOp On the{001}sur-faces and cOnseguently the<111>pOlarity Of Zinc-blende structure is reflected in the etching behaviOr Of these surfaces.Like mOst Of the I-V cOmpOund semicOnductOrs c-GaN has tWO types Of{111}sur-faces(111)A and(111)B surfaces.The tWO types Of surfaces resulting frOm the crystallOgraphic pO-larity Of the<111>directiOns exhibit markedly dif-ferent physical and chemical prOperties.It Was fOund that the(111)B surfaces are far mOre reac-tive than the A surfaces in mOst chemical a-gents[15 16].Grabmaier et al[15].etched GaAs utiliZ-ing mOlten K0~they fOund mOlten K0~etchant attacks the(111)B surfaces mOre rapidly.GatOs et al[16].investigated the etching behaviOr Of the (110)and(001)surfaces Of InSb.The
reactivity Of the principal crystallOgraphic planes Of InSb de-creases in the fOllOWing Order.
(111)B>(001)>(110)>(111)A
We assume this Order Of reactivity can alsO be ap-plied tO the present case therefOre the(111)B sur-faces are mOre chemically reactive than the(111)A surfaces.The difference in reactivity Was cOnsid-ered the cOntrOlling factOr fOr the differences in etching behaviOr betWeen the tOW types Of sur-faces[16].TherefOre Once small pits With{111} facets fOrm at the defect sites in the initial stage Of the Wet etching they Will gradually develOp intO larger rectangular pits.The preferential etching Of (111)B surfaces is shOWn schematically in Fig.3. The etch figures grOW in siZe and merge intO each Other preferentially in the[110]directiOn due tO the faster etching Of(111)B surfaces as shOWn in Fig.2(b)and(c
)
.
Fig.3Schematic diagram Of K0~based sOlu-
tiOns prOducing etch pits On(001)surface Of
GaN epilayer grOWn On GaAs(001)
In additiOn K0~agueOus sOlutiOn Wet etching might be a useful methOd fOr evaluating the surface defects Of c-GaN epilayers.Steps bOth in[110]and [110]directiOns can be Observed in Fig.2(c).
017半导体学报23卷
These steps probably correspond to the intersec-tion of stacking faults lying on{111}planes With the(OO1)surfaces of Zinc-blende GaN.Figure4 shoWs a plan-vieW TEM(PV-TEM)image of the same GaN film near the top surface.Bands com-posed of black-White fringes can be observed in this figure Which are formed by inclined stacking faults in the PV-TEM observations[17].It is noteWorthy that the distribution and density of the striations in Fig.2are very similar to that of the stacking faults in Fig.4.Therefore We conclude that KO~ague-ous solution Wet etching can probably be used to reveal surface defects of c-GaN epilayers groWn on GaAs
.
Fig.4PV-TEM micrograph of as-groWn GaN epi-
layer near the surface The dark-blight fringes orig-
inate from the inclined stacking faults in the GaN
epilayer.
电动拖布
Conclusion
We have reported on the etching characteris-tics of GaN epilayers groWn on GaAs(OO1)sub-strates by LP-MOVPE.That KO~-based solutions etch rectangular parallelogram pits rather than sguare pits on the sample surfaces indicates asym-metric etching behavior in the tWo different<11O> directions.The asymmetry Was attributed to the different reactivity betWeen(111)A and(111)B planes.(111)B planes are more chemically reactive than(111)A planes resulting in the elongated etchfigures on the GaN surfaces.In addition We noticed the similarity betWeen the step distribution of the sample etched in SEM images of hot KO~ agueous solution and the distribution of the stack-ing faults revealed by PV-TEM observation. Therefore the etching of KO~agueous solution can probably be used to reveal surface defects of GaN/GaAs(OO1)system.
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7期Shen Xiaoming et al.:Etching Behavior of GaN/GaAs(OO1)Epilayers GroWn by
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